[Gluster-devel] Gluster tiering feature

[Dan Lambright]

----- Original Message -----
> From: "Lalatendu Mohanty" <lmohanty at redhat.com>
> To: "Dan Lambright" <dlambrig at redhat.com>, "Gluster Devel" <gluster-devel at gluster.org>
> Sent: Monday, October 20, 2014 11:28:28 AM
> Subject: Re: [Gluster-devel] Gluster tiering feature
> 
> On 10/18/2014 01:11 AM, Dan Lambright wrote:
> > Myself, Joseph Fernandez, and others have been working on a tiering feature
> > for gluster. We are in the prototype phase and demoed some code recently
> > internally to Red Hat. In its first incarnation it resembles the server
> > side cache tier Ceph has, or dm-cache in the kernel. So, fast storage and
> > slow storage are exposed as a single volume, data that is frequently used
> > makes its way to fast storage, and the system responds dynamically to
> > changing usage. Because migration between tiers is time consuming, a cache
> > tier is a good fit for workloads where the set of hot data is stable. A
> > cache tier can be added or removed at run-time. The tiering logic is very
> > general-purpose infrastructure, and can be used for elaborate data
> > placement graphs or other data migration features.
> >
> > The design is in early stages.. but current thinking can be found on the
> > feature page and the links below, and all the code is on the forge.
> >
> > http://www.gluster.org/community/documentation/index.php/Features/data-classification
> >
> > goo.gl/bkU5qv
> >
> > Thanks,
> > Dan
> > _______________________________________________
> > Gluster-devel mailing list
> > Gluster-devel at gluster.org
> > http://supercolony.gluster.org/mailman/listinfo/gluster-devel
> 
> Looks awesome!! Thanks for putting these docs.
> 
> However I have a few questions on the following.
> /"Current thinking is a snapshot cannot be made of a volume with a cache
> tier, however a cache may be "detached" from a volume and then a
> snapshot could be made//"./
> 
> As far as I can understand, detaching cache tier will take a good amount
> of time (of course it depends on the amount of data), so detaching the
> cache tier and then taking snapshot will be time consuming and might not
> be used much (from a user point of view) . However I am wondering if
> pausing cache is less expensive then detaching cache? Also does pause
> cache migrates all data from hot to cold subvol, or just from that time
> onwards?
> 
> Thanks,
> Lala


yes, the thinking is "pausing" the cache would demote all the data off the fast tier.

I think getting snap to work with this is a stretch goal, at least in the beginning. I'm worried about thorny issues that may happen. For example, suppose someone snapped a tiered volume, then detached the fast tier, the tried to restore the volume. 


>